Heat pipes



Dec. 3, 1968 P. FIEBELMANN 3,414,475

' HEAT PIPES Filed May 13, 1966 3 Sheets-Sheet 1 FIG 1 Dec. 3, 1968 P. FIEBELMANN 3,414,475

HEAT PIPES Filed May 13. 1966 5 Sheets-Sheet 5 United States Patent 3,414,475 HEAT PIPES Peter Fiebelmann, Besozzo, Italy, assiguor to European Atomic Energy Community (Euratom), Brussels, Belgium Filed May 13, 1966, Ser. No. 550,029 Claims priority, application Germany, May 20, 1965, E 29,347 13 Claims. (Cl. 176-72) ABSTRACT OF THE DISCLOSURE A tubular heat pipe comprising an evaporization zone, with an internally mounted fissile heat source, and a condensation zone, with external secondary cooling means. The pipe is at least partially filled with a liquid coolant which evaporates in the ev'aporization zone and returns, after condensing, along the inner surface of the pipe walls to the evaporization zone. A circular gap is formed between the tube wall and the heat source with a capillary inlet.

The invention relates to heat pipes. A heat pipe is a transfer device comprising a container, condensable vapour, and capillary means disposed within the container capable'of causing the transport of the condensed vapour from a cooler area of the container to a hotter area. The transport of the vapour through the container uses, as the driving force, the difference in vapour pressures in the high temperature zone and the cold temperature zone. The liquid which condenses in the cold zone is returned to the evaporation zone by capillary action. 'Ihus, fluid circulation is established in the pipe with the non-heated end of the pipe acting as a condenser. By means of this circulation, a heat flux is created to flow from the heated end of the pipe to the non-heated end of the pipe. Such heat pipes are described by Grover, Cotter and Erickson, Structures of Very High Thermal Conductance, 35 Journal of Applied Physics, 1990/91 (June 1964).

It is very important in the operation of heat pipes to ensure that an adequate supply of condensate reaches the evaporation space. If the supply is interrupted for any reason, the pipe becomes unserviceable and may even be destroyed.

The main risk in this respect lies in the known manner of returning the condensate to the evaporation zone, for the condensate always enters the evaporation zone from the edge thereof, and so the pressure in such zone is the main factor in determining whether or not the (externally) heated surface of the evaporation zone is completely wetted with condensate. Above a particular heating power orwhich is the same thingbeyoud a particular length of evaporation zone, it becomes impossible to ensure that condensate is supplied as far as the pipe base (i.e. the end of the pipe remote from the evaporation zone). The remote walls of the evaporator therefore dry out.

The invention seeks to solve or reduce this problem by separating the evaporation zone from the condensate arrival zone. In practical terms, this means that the condensate entering the evaporation zone is kept away from the heated evaporator surface but is subsequently supplied uniformly to the whole of such surface.

The invention accordingly provides a heat pipe in which the condensate returning to the evaporation zone travels on one side of a capillary structure and then passes through the structure to the other side where it is evaporated.

In one embodiment of the invention, the evaporation zone is internally heated and the returning condensate passes first behind a capillary partition which separates the evaporation zone and the heat source from the remainder of the space in the tube and through which the condensate flows into the evaporation zone by capillary action.

If a heat pipe is combined with a heat source in the form of a nuclear fuel element, the source may be a fissile bar disposed in the evaporation zone symmetrically of the axis thereof, and the capillary partition may be a concentric porous tube. This embodiment has already been tested satisfactorily with an electric heater simulating the nuclear fuel rod. In this way up to several metres length of rod can be cooled by heat pipes, for example under conditions of weightlessness in outer space.

In a second embodiment of the invention, to provide a separation between the evaporation zone and the condensate arrival zone, the evaporation zone is heated at one side, the condensation zone for example is cooled at one side or on all sides, and the condensate returning to the evaporation zone passes through a capillary structure at a location apart from the heated side. The capillary structure may leave an evaporation zone free internally but provide all the external wall parts of the evaporation zone-the heated as well as the unheated surfaces.

Particulars of the embodiments referred to and the operation of the corresponding heat pipes will now be described by Way of example, in greater detail with reference to the accompanying drawings wherein:

FIGURE 1 is a view in vertical section of a heat pipe which is heated centrally by a nuclear fuel rod in the evaporation zone;

FIGURE 2 is a partial view showing a variant of FIG- URE 1;

FIGURE 3 is a diagrammatic view in longitudinal section of a heat pipe which is heated externally at one side;

FIGURE 4 shows a first variant of the heat pipe of FIGURE 3;

FIGURE 5 shows a second variant of the heat pipe of FIGURE 3, and

FIGURE 6 is a view, in a section perpendicular to the longitudinal axis, of a heat pipe unit which is heated centrally by a nuclear fuel rod and which has a number of evaporation zones.

Referring to FIGURE 1, there can be seen a heat pipe 1, a heat source 2, a capillary tubular liner or insert 3, for instance, of sintered ceramic material, a gas tight tubular top member 4, a capillary condensate return structure 5, a cooling water system 6, entry and exit pipes 7, 8 thereof, a sealing bush 9, an associated screwed plug 10 and a co-operating ring gasket 11. The device is arranged for vertical operation. A number of devices of this kind can be combined to form a reactor rod lattice, in which event the heat source 2 would, as shown, be a nuclear fuel rod.

The heat pipe part encircled by the cooling system 6 forms the condensation zone of the heat pipe, and the heat-receiving part of the heat pipe forms the evaporation zone thereof. FIGURE 1 does not show the heat carrying vehicle which, when a nuclear fuel element is used as heat source, is a meltable and ev-aporable metal. When the heat pipe is off, the heat vehicle is in the collecting space 12; when the pipe is started up, the heat vehicle in the space 12 is evaporated therefrom by auxiliary heating in the heat pipe. In operation there is a two-phase circulation of the heat vehicle-a vapour phase which rises along the heat pipe axis, and a liquid phase which flows down the pipe wallbetween the condensation zone and the evaporation zone. The evaporation zone of the heat pipe is the annular space 13 between the heat source 2 and the capillary liner 3.

The outer ring space 14 is devised to form a condensate reservoir, for the separation between the evaporation zone and the condensate supply zone is achieved by the evaporation zone-in this case the annular space 13being internally heated-Le. by the heat source 2 and by the returning condensate first going behind a partitioni.e., the liner 3which separates the vapour space and the heat source from the remainder of the gas space of the pipe and through which the condensate can flow into the evaporation zone by capillary action. Consequently, the returning condensate entering the evaporation zone is not heated to such an extent as wholly to evaporate before reaching the bottom of the tube. Another advantage of the ring space 14 is that condensate is supplied therefrom uniformly and to all places of the evaporator surfacei.e., the inside surface of the porous liner 3.

The function of the top member 4 on the liner 3 is to provide a Communication pressurewise between the ring space 14 and the condensation zone, in order that the vapour pressure drop between the evaporation zone and the condensation zone may be fully used to supply the evaporator surface with condensate. If the member 4 is of substantial length, it should be heat-insulated. The other condensate return structure 5, in the form of a number of wires whose axes are parallel to one another and which extend into the condensate collecting chamberacts as a capillary structure and helps to even out the condensate return.

When the heat pipe is operated vertically in a gravitational field, the width of the gap between the porous liner 3 and the heat pipe wall does not affect the return movement of the condensate since the condensate descends to the evaporation zone by gravity. The gap need not therefore be capillary, and so the annular space 14 acts purely as a condensate collecting space.

On the other hand, when the heat tube is operated horizontally or in a position inclined to the horizontal in a gravitational field and when the tube is operated in weightless space, the gap must either be a capillary gap over its whole 1engthi.e., it must act as a capillaryor it must have a capillary constriction at least at the top end of the porous liner, for in these cases the condensate return must be produced and maintained by capillary action. Consequently, in this case too, the pressure difference between the evaporation zone and the condensation zone is used. If the space 14 is connected to the condensation zone by a capillary zone only at the top end, the wider part of the space 14 lower down is merely a condensate collecting space. The heating of the heat pipe must be so devise-d that the level of condensate at least into the constriction is sufiicient for capillary connection. The annular space 14a between the impervious top member 4 and the heat tube wall can be used for capillary connection of the ring space 14 provided that the condensate extends into this ring space or gap.

In all cases, the effective capillary widths at the various places of the heat pipei.e., those liquid surfaces of the capillary gaps and passages which are exposed to the gas spacemust, from the condensation zone to the evaporation zone of the tube, at least stay constant and will preferably decrease substantially steplessly. In FIGURE 1, therefore, the smallest gap between the wires of the condensate return structure 5 must be larger at the top end of the condensation zone than any capillary connection gap which there may be in the annular space between the top member 4 or liner 3, and the latter g-ap must in turn be larger than the pores of the liner 3 which, as already mentioned, have a capillary action. The theoretical capillary ceiling of a capillary, and the delivery of condensate to the evaporation space, vary in inverse proportion to effective capillary width.

If the space 14 is devised as a condensate collecting space, there is a chance, if the heat vehicle used is water or a similar thinly viscous liquid, that, more particularly when the heat pipe is out of operation, the condensate in the ring space 14 may discharge slowly through the pores of the liner 3 into the collecting space 12, with the result that at starting or when the pipe is heated up abruptly from low-load conditions, too little, if any, condensate is initially present in the porous liner.

As a constructional way of obviating this disadvantage, FIGURE 2 shows how an impervious partition 15 can be inserted into an annular space 14 devised as a condensate collecting space, a capillary gap 16 being left between the partition 15 and the porous liner 3. The partition 15 extends over the whole length of the porous insert 3 and is spaced apart therefrom by means of pimples or the like on the inside. Gaps 17, 18 are left at the top and bottom ends of the partition 15, the bottom gap 17 being a condensate entry gap while the top gap 18 gives communication with the gas space of the pipe. The maximum level of condensate in the space 1 4 is the top edge of the partition 15. Even when the condensate level drops as far as the bottom of the annular space 14, the gap 16 still remains full of condensate, so that the porous insert receives a supply of condensate in all conditions of operation. This feature is very useful in cases where the heat pipe output varies considerably, for despite variations in condensate level, the porous liner is always supplied uniformly with condensate.

FIGURE 3 shows an embodiment of the invention wherein the effect of the porous liner hereinbefore described is produced by a stack of annular rings 19 and wherein, to improve further the separation between the evaporation zone and the condensation flow zone, the evaporation zone is heated only at one side and, correspondingly, the condensation zone is cooled only at the opposite side, as indicated by arrows f and 1 at the evaporation zone and condensation zone respectively of a heat pipe 20. The returning condensate in the evaporation space goes by capillary action between the rings 19 from the unheated wall to the heated wall, as indicated by arrows f The rings are formed with spacing pimples or protuberances or the like, for stacking at a capillary spacing, and also bear laterally against the pipe wall by way of similar protuberances or the like. The spacing of the rings is such, at the opposite end of the heat pipe to the heating zone (area framed by chain lines) that, as considered over the whole group of rings, a condensate collecting space or a capillary space having the function described with reference to FIGURE 1 is provided. In FIG- URE 3 the condensate is represented by a line of vs 21. The spacing of the rings is such that the gaps between the rings have a capillary action. The pipe vapour space is formed by the cumulative effect of all the apertures bounded by the rings, as indicated by arrows f A gastight tubular top member 22, which has been described with reference to FIGURE 1, and .a condensate return structure 23, are provided in the central and top part respectively of the heat pipe.

Heat pipes having a stack of rings can have the variants, hereinbefore discussed with reference to FIGURE 1 of the annular space between the stack and heat pipe (capillary connection in the special cases of horizontal or substantially horizontal operation of the heat pipe in the gravitational field or of the tube operating in any position in conditions of weightlessness). In heat pipes having a stack of rings, the effective capillary width can readily be devised to decrease downwards in the manner shown in FIGURES 4 and 5. The first difference between FIG- URE 4 and FIGURE 3 is that heat pipe 24 of FIGURE 4 is filled right up with loose rings 25, so that manipulation is simpler than for the embodiment shown in FIGURE 1. The between-rings spacings or gaps 26 decrease gradually from the condensation zone to the evaporation zone, to which end the spacing protuberances are of correspondingly changing size. Consequently, and as is required, the suction of the horizontal capillaries which are devised in this way with different widths decreases downwards. Arrows f f represent the supply and removal of heat and arorws f represent the flow of condensate. The vertical annular space 27, which is wider at the condensation end than at the evaporation end, can be devised either as a capillary or just as a condensate collecting space. In the latter case, and in contrast to FIGURE 1, no capillary contact constriction is necessary since a capillary connection between the space 27 and the heat pipe gas space is provided by the between-rings gaps. Its condensate space therefore represents merely the hydraulic transmission path in the system of communicating gaps.

FIGURE 5 shows how the effect of variation in spacing can be achieved without altering the spacing between the rings. Rings 28 are placed freely one above another with the interposition of spacing protuberances of the same height, horizontal capillary gaps 29 being left between individual rings. On their inner edges near the vapour space, the rings 28 are bevelled conically. Wedge-shaped ring grooves 30 of uniform depth are left; when the pipe is in operation theg rooves 30 fill up to dilferent levels with condensate in dependence upon pressure distribution in the gas space, the level of the condensate in the grooves 30 decreasing downwardly. At the groove base the capillary gaps are so devised as to be just full of condensate in the evaporation zone when maximum heat output is being transmitted, the conical groove part above having just had all the liquid evaporated from it. In all, the embodiments shown in FIGURES 4 and 5 represent self-regulating hydraulic supply systems. A continuous helically extending wire spiral can be used as capillary insert or liner instead of separate rings or segments.

FIGURE 6 shows how heat pipes having a stack of rings or segments for a nuclear fuel element can be used in practice. A fissile rod 33 is inserted into an inner tubular member 31 having four radial ribs 32. The ribs divide into four sub-chambers an annular chamber bounded by an outer covering pipe 34 which is the actual heat pipe. Each sub-chamber forms an independent evaporation and condensation system of the kind shown in FIGURE 3, a central heat source of the kind shown in FIGURE 1 being used for all the evaporation spaces. Combining the two principles optimises surfaces and volumes so far as thermal conditions and nuclear engineering are concerned. The volume ratio of fissile to non-fissile material can be so devised for prismatic elements that even when the element is used in fast reactors criticality is reached with an adequate reserve of reactivity.

As in FIGURE 4 or 5, stacks of rings 35 having lateral and bottom spacing protuberances are introduced into the four working spaces of the element. The between-rings spacing are such that the horizontal gaps form capillaries of graded width. At the periphery of the element they leave free on the inside a gap 36 which is wider than a gap 37 at the fuel rod 33. The gap 36 forms the condensate return space, and condensate is evaporated via the gap 37 into the vapour spaces 38, the condensate going from the outer ring arcs via the radial ring parts to the inner ring arcs.

If required, the ring stacks can be replaced by a single porous capillary member corresponding in crosssectional shape to the tubular members 34, 31 with radial ribs 32--i.e., a capillary member which is double-walled and has radial partitions. Such a member would be inserted into the ring space between the outer tube 34 and the inner tubular member, which in this case is unribbed.

For very dense packing of elements of the kind shown in FIGURE 6 into a reactor rod lattice, the various items concerned are, conveniently, prismatic, e.g. hexagonal. For temperature equalisation along the fuel rod, each heat pipe system can have horizontal partitions like tiers-i.e., in sections, each section operating axially as a separate heat pipe, so that the effect of temperature constancy is utilised zonewise in evaporation and condensation.

What I claim is:

1. A tubular heat pipe comprising a tubular member having an evaporation zone and a condensation zone, a fissile=heat source mounted within said member in said evaporization zone, external secondary cooling means adjacent said condensaiton zone, a coolant liquid filling at least part of said tube, said coolant being evaporated in the evaporization zone and after condensing returning along the inner surface of the walls of said pipe member to the evaporization zone, capillary inlet means formed in an annular gap formed between the walls of said tube and said heat source, said capillary inlet extending at least along the total height of said heat source and allowing the coolant liquid to traverse said gap to approach said heat source.

2. A heat pipe according to claim 1 in which said capillary inlet comprises a tube of porous material having an outer diameter less than the inner diameter of said tube andan inner diameter greater than the outer diameter of said heat source.

3.- A heat pipe according to claim 1 in which said capillary inlet comprises a plurality of stacks of metallic rings, each stack filling a segment of said gap, each ring being substantially perpendicular to the longitudinal axis of said pipe and being spaced from the adjacent rings by a capillary distance.

4. A heat pipe according to claim 1 in which said capillary inlet comprises a plurality of tubes of porous material, each said tube filling a portion of said gap and having its longitudinal axis parallel to the longitudinal axis of said plpe.

5. A heat pipe according to claim 1 in which said capillary-inlet is closer to said heat source than said pipe member forming a space larger than a capillary gap between said inlet and said pipe, said space being susbtantially completely filled with coolant liquid under operating condi tions.

6. A heat pipe according to claim 2 in which a metallic, non-porous tube is hermetically mounted on said tube of porous material and forms an extension thereof in the direction of the condensation zone.

7. A heat pipe according to claim 3 in which a metallic, non-porous tube is hermetically mounted on said stacks and forms an extension thereof in the direction of the condensation zone.

8. A heat pipe according to claim 3 in which the segments are hermetically separated from one another by radial walls.

9. A heat pipe according to claim 4 in which the tubes are hermetically separated from one another by radial walls.

10. A heat pipe according to claim 3 in which said stacks of rings extend substantially over the entire length of said pipe.

11. A heat pipe according to claim 3 in which the spacing between the rings decreases from the condensation zone towards the evaporization zone.

12. A heat pipe according to claim 3 in which said rings are uniformly spaced.

13. A heat pipe according to claim 3 in which at least one annular edge of said rings in the evaporization zone are bevelled.

References Cited UNITED STATES PATENTS 3,229,759 1/ 1966 Grover 105 3,305,005 2/1967 Grover et al. 165-105 FOREIGN PATENTS 340,281 9/1959 Switzerland.

ROBERT A. OLEARY, Primary Examiner.

ALBERT W. DAVIS, JR., Assistant Examiner. 

